It is easy to vote 88% with your party because 80% of the votes are irrelevant to anything that matters.
The more relavant % to calculate is, how many of his positions on issues are conservative (roughly Republican) or liberal (roughly Democrat). As Captain Virgil Hilts points out, many of his positions on issues are anything but conservative. In fact, he seems to have taken great glee in poking his finger in the eye of his own party on many occasions, preferring to cave in to the opposition rather than standing on "conservative" principle.